Name the three acoustic variables.

• A. Pressure, Velocity, Distance
• B. Pressure, Density, Distance
• C. Pressure, Temperature, Density
• D. Elasticity, Density, Viscosity

Answer: (B)

Acoustic waves are described by three quantities that capture how the medium is perturbed: pressure, density, and particle velocity. When a wave passes, it creates pressure fluctuations as regions are compressed and rarefied, it causes tiny changes in density as the medium is momentarily squeezed or stretched, and it sets the particles into back-and-forth motion, which is described by particle velocity. These three variables together characterize the instantaneous state of the sound field in the medium.

Distance is a spatial measure, not an intrinsic wave variable, so it isn’t listed as one of the acoustic variables. Temperature, while related to the medium, is not what we use to describe the wave itself in standard ultrasound analysis. Elasticity (bulk modulus) and viscosity are material properties that influence how waves propagate but are not the immediate acoustic quantities describing a given wave at a point in time.